1. Field of Invention
This invention relates to a cutting element comprising a polycrystalline diamond portion which is bonded between two metal portions. Such cutting elements are commonly shaped and attached to the shaft of a drill bit and are used in drilling holes or other machining operations.
Throughout the following disclosure, the phrase "polycrystalline material" or "polycrystalline diamond" is intended to cover all super abrasion-resistant polycrystalline materials including, but not limited to, polycrystalline diamond, polycrystalline cubic boron nitride, polycrystalline wurtzite, boron nitride and combinations thereof. For convenience, both high pressure forms of boron nitride will be referred to as CBN.
Throughout the following disclosure the phrase "soft metal" is intended to cover a material with a Young's Modulus less than approximately 45.times.10.sup.6 psi and is selected from the following group consisting of cobalt, nickel, iron, copper, silver, gold, platinum, palladium, alloys of these metals and intermetallic compounds containing these metals.
2. Prior Art
The prior art is replete with cutting elements formed or shaped from various metals. These cutting elements are traditionally used on tools such as rock drills or drills for machining operations which function with a simultaneous cutting and wedging action. For example, such cutting elements are used in flat drills which comprise a substantially flat portion and a rotatable shaft with a central axis.
In its simplest form the traditional cutting element of a flat drill has planar top portions which are non-perpendicular to the central axis of the shaft. The top of the shaft has four surfaces sloping up from the perimeter surfaces of the shaft towards the central axis, a slit which extends across the top of the shaft and is adapted to receive the cutting element and hold it in a fixed position for the drilling operations.
In these prior art cutting elements the substantially flat portion or blade member has two opposing pentagonal faces. At the top of the substantially flat portion is a chisel edge which is perpendicular to the central axis of the rotatable shaft. The midpoint of the chisel edge intersects the central axis of the shaft. A vertical plane passing through the chisel edge intersects the vertical plane passing through the lines where the pairs of sloped surfaces meet, usually at a non-perpendicular angle. The top end of the flat portion also has two other cutting edges known as the lips of the drill which extend from each end of the chisel edge and slope downward and away from the central portion of the rotatable shaft.
Each lip provides the leading edge to a cutting lip surface which slopes down from the lip and across the top of the substantially flat portion. The pairs of sloping surfaces of the rotatable shaft are positioned below the top of the substantially flat edge portion, thus exposing part of the pentagonal faces of the substantially flat portion. The cutting lip surfaces terminate at the chisel edge.
The flat portion has two additional cutting edges, called the margins which form the leading edges of margin surfaces at the sides of the substantially flat portion. Each margin is substantially parallel to the central axis of the rotatable shaft and extends downward from the lip along an edge of the pentagonal face. The margin and lip meet at the peripheral corner of the flat portion. These margin surface are substantially parallel to each other.
Thus the flat portion has five cutting edges, the chisel edge, two margins and two lips. Each pentagonal face of the flat portion has a margin and a lip and the central end points of the lips provide opposite end points of the chisel cutting edge.
As the drill rotates about its axis, it is forced against a workpiece. As the drill contacts and begins to penetrate the workpiece, the chisel edge is subjected to compressive forces. This results in a wedging or chiseling action whereby the workpiece material displaced by the drilling action moves toward the outer ends of the lips. As this occurs, the lips begin to cut into the workpiece and remove chips or fragments of the workpiece. This cutting action subjects the lips to torsional forces. When a drill is new or recently resharpened, the margins perform no or little cutting action. The peripheral corners, however, remove more material than the inner portions of the lips. As a result, after continued use of the flat drill, the peripheral corners become somewhat rounded. After the peripheral corners where the margin meets the lip begin to wear, the margins increase their function as a cutting edge and the ability of the lips to cut is reduced.
As the lips continue to wear, the compressive forces of the drill must be increased to maintain the ability of the drill to penetrate the workpiece. With workpieces where the drill passes through and out the back side of the workpiece, the dulling of the drill causes burrs or frays at the exit hole of the workpiece as well as requiring increased drilling forces. When the additional drilling forces required reach a predetermined level or where exit holes on workpieces become frayed, it becomes necessary to terminate drilling operations and replace or resharpen the flat drill. The flat drill is usually resharpened by machining the cutting lip planes of the substantially flat portion until the rounded peripheral corners are eliminated. When the cutting lip planes have been machined down to the point where the margins are too short, the drill is no longer an effective cutting element and is discarded.
Recently, the machining of both harder materials as well as more abrasive materials has increased which has introduced a great need for drills which can withstand machining such materials. The use of cutting tools made from wear resistant materials such as polycrystalline diamond or polycrystalline cubic boron nitride to machine harder or abrasive surface has been disclosed. (See U.S. Pat. No. 3,745,623 for Diamond Tools for Machining issued to Robert H. Wentorf, Jr. on Dec. 27, 1971).
In addition to the above tools, non-planar diamond surfaces with an underlying carbide substrate which provides a backing for the diamond layer have been disclosed. (See U.S. Pat. No. 4,109,737 for Rotary Drill Bit issued to Harold Bovenkerk on Aug. 29, 1978 and U.S. Pat. No. 4,333,540 for Cutter Element and Cutter for Rock Drilling issued to William Daniels and John Cheatham on June 8, 1982). These diamond surfaces are in the shape of a dome or wedge and are used for elements in rock drills where they encounter forces that are substantially normal or perpendicular to the diamond-carbide interface. In each of these prior art devices the polycrystalline diamond surface is supported by the carbide structure against forces applied against the polycrystalline diamond.
A recent improvement in cutting elements overcomes many structural problems with the above described prior art and is disclosed in co-pending application Ser. No. 464,401 filed on Feb. 7, 1983 in the names M. Duane Horton and L. Brent Horton and assigned to the same assignee as the present application and which is incorporated herein by reference. This improved cutting element for use in a flat drill has cutting edges which are comprised of polycrystalline diamond or the like mounted to a cemented carbide substrate or a similar hard material which is held by a rotatable shaft with a central axis. The polycrystalline material is mounted to a cemented carbide substrate (typically cemented tungsten carbide) and it is unsupported with respect to torsional forces exerted upon it during drilling. This shaft is adapted for insertion into a drilling machine.
The cutting element is in the form of a substantially flat portion or blade member for a flat drill. The rotatable shaft has four shaft surfaces sloping up from the perimeter of the shaft towards the central axis. A slit through the top of the rotatable shaft divides the sloping shaft surfaces into two pairs of shaft surfaces. This slit holds the substantially flat portion which comprises a cemented carbide substrate having polycrystalline material attached to its top. The carbide substrate has two opposing substantially parallel pentagonal faces. Located at the top of the substrate is an edge which may be rounded and is perpendicular to the central axis of the shaft. This edge is also the intersection of two sloping surfaces located on top of the substrate. Polycrystalline material such as polycrystalline diamond is mounted to both of these substrate surfaces. The leading edge of each polycrystalline diamond coated surface is called a cutting lip edge which lies in a plane which is parallel to the substrate. The cutting lip surfaces meet to form the chisel edge which is located at the uppermost part of the substantially flat portion. The thickness of the diamond at the outermost end of the lip forms a cutting edge called the margin. This flat drill is resharpened as described above by machining the cutting lip planes of the substantially flat portion until the rounded peripheral corners are eliminated. A problem with this cutting element is that the diamond cutting edges are relatively thin and since it is not possible to economically make thick polycrystalline diamond, the number of resharpenings are limited in number.
The substantially flat portion is attached to the rotatable shaft, which is in turn inserted into a drill. When the drill is rotated about its central axis and the cutting edges are forced against a workpiece, the polycrystalline material is unsupported with respect to the torsional forces of drilling.
Another significant problem in the above cutting element resides in the polycrystalline diamond to substrate bond. The cutting element before shaping to the structures described above is formed in a high pressure press. In the formation process cemented tungsten carbide is generally used as a substrate or platform upon which diamond starting material is placed and the combination is then subjected to high temperatures and pressure for a period of time sufficient to form or grow the polycrystalline diamond directly upon the substrate. This formation process is well known in the art and further description herein is deemed unnecessary. In this formation process the interface between the polycrystalline diamond and the cemented tungsten carbide substrate is in a substantially unstressed condition.
After formation the polycrystalline diamond is bonded directly to the cemented tungsten carbide substrate. However, during cooling the cemented tungsten carbide with a much greater coefficient of thermal expansion compared to the polycrystalline material tends to shrink or compress more than the diamond. This difference in the coefficient of thermal expansion leaves the cemented tungsten carbide material stretched and the polycrystalline diamond under compression at the diamond/cemented carbide interface. This condition thus creates internal stresses in the diamond/cemented carbide interface. These stresses may be sufficient to actually cause either fracturing in the cemented tungsten carbide material or in the polycrystalline diamond portion of the compact structure, or delamination, a separating of the materials at the diamond/carbide bond. If either the fracturing of the polycrystalline diamond or cemented tungsten carbide, or delamination is significant the resulting compact structure is useless and must be considered as scrap, thereby reducing the yield of the manufacturing process.
Another form of compact structure which is a multiple layer structure having a polycrystalline diamond center layer bonded between two layers of tungsten or cemented carbide. In this multiple layer or sandwich compact structure the internal stresses in each polycrystalline diamond/tungsten or diamond/carbide interface are increased in comparison to the stresses in the two layer compact structure discussed above. In the multiple layer compact structure during cooling both tungsten or carbide layers shrink or compress at a different rate than the polycrystalline diamond and exert increased forces at the bonding interfaces. Since the polycrystalline diamond layer does not have a free surface similar in size and bonding surface as exists in the two layer compact it cannot yield to compensate for the increased internal stresses. As a result both fracturing in the tungsten or carbide layers and in the polycrystalline diamond layer, and delamination are increased. Thus, it is difficult to manufacture a multiple layer or sandwich compact structure and yield for such manufacturing operation is quite low.
Even if the resulting multiple layer compact structure does not have flaws sufficient to reject the product, subsequent bonding of the compact to the drill shaft increases these internal stresses and the possibility of failure. For example, the subsequently shaped multiple layer compact structure forming the cutting element is mounted on the tool, e.g., shaft by brazing or similar techniques. In the brazing technique the tungsten or diamond/carbide portion of the multiple layer compact, and the shaft are heated to a sufficient degree to induce bonding. During the subsequent cooling period, the internal stresses in the polycrystalline diamond/tungsten or diamond/carbide bonds created during the formations of the multiple layer compact structure are increased. The increased mass of material, namely the tungsten or carbide portion of the compact now attached to the shaft metal produces more stress after the brazed assembly is cooled due to the differences in the thermal contraction. The increased thermal contraction differential causes still increased stresses in the polycrystalline diamond/tungsten or diamond/carbide bonds and their increased stresses cause both increasing fractures in the polycrystalline and tungsten or carbide layers and delamination. These defects may be pronounced enough to cause an immediate failure which results in a rejected part. In some instances the increased stress and resulting problems simply weaken the assembly so that the useful life of the resulting tool is reduced.
3. Objects of the Invention
It is a general object of the present invention to alleviate the aforementioned problems. It is another general object of the present invention to provide a cutting element having a polycrystalline diamond material portion attached to a soft metal portion with reduced stresses in the element that result from the soft metal portion and polycrystalline material portion having different coefficients of thermal expansion.
It is a specific object of the present invention to provide a cutting element for machine operations comprising a polycrystalline material portion attached to at least one side portion or face with the side portion being made from a material having a Young's Modulus less than approximately 45.times.10.sup.6 psi. It is yet another specific object of the present invention to provide a cutting element for machine operations comprising a polycrystalline material portion sandwiched between two side portions at least one of the side portions being made from a material having a Young's Modulus less than approximately 45.times.10.sup.6 psi. It is still another specific object of the present invention to provide a cutting element for machine operations comprising a polycrystalline material center portion sandwiched between two side portions, each side portion being made from a material having a Young's Modulus less than approximately 45.times.10.sup.6 psi.
It is another specific object of the present invention to provide a cutting element for machine operation comprising a polycrystalline material portion attached to at least one soft metal portion with the soft metal portion being made from a material selected from the group consisting of cobalt, nickel, iron, copper, silver, gold, platimum, palladium, alloys of these metals and intermetallic compounds containing these metals. It is another specific object of the present invention to provide a cutting element for machine operations comprising a polycrystalline material center sandwiched between two side portions with at least one of the side portions being made from a material selected from the group consisting of cobalt, nickel, iron, copper, silver, gold, platimum, pallidum, alloys of these metals and intermetallic compounds containing these metals. It is yet another specific object of the present invention to provide a cutting element for machine operations comprising a polycrystalline material center sandwiched between two side portions with each side portion being made from a material selected from the group consisting of cobalt, nickel, iron, copper, silver, gold, platinum, palladium, alloys of these metals and intermetallic compounds containing these metals.